Known processes for producing an ether-imide type bismaleimide, e.g., 2,2-bis[4-(4-maleimidophenoxy)phenyl]propane, typically include a dehydrating cyclization reaction of a corresponding maleamic acid using a dehydrating agent (e.g., acetic anhydride) in a ketone solvent (e.g., acetone) or a polar solvent (e.g., N,N-dimethylformamide) in the presence of a base (e.g., triethylamine) and a metallic compound (see U.S. Pat. No. 4,460,783). However, this process does not achieve sufficient progress of the cyclization reaction, only to provide a bismaleimide containing a considerable amount of uncyclized maleamic acid. Therefore, while the resulting bismaleimide is soluble in organic solvents, such as acetone, toluene, methyl ethyl ketone, etc., it liberates water on curing due to the uncyclized maleamic acid and other impurities such as an addition product between the bismaleimide and acetic anhydride, and resulting cured product suffers from voids or blisters or has a reduced glass transition temperature and so deteriorated heat resistance.
R. H. Dahmas has obtained a high purity and substantially acetone-insoluble ether-imide type bismaleimide by dissolving the above-described imidation product in acetone followed by selective re-precipitation and proposed an acetone solution of a mixture of the resulting high purity acetone-insoluble ether-imide type bismaleimide, a corresponding precursor maleamic acid, and a polyamine of an amount required for neutralizing the maleamic acid as a composition suitable for a bismaleimide resin matrix composite (see U.S. Pat. Nos. 4,808,646, 4,816,512 and 4,924,005).
On the other hand, it is also known to prepare a bismaleimide by a process comprising addition reaction between an aromatic diamine corresponding to an ether-imide type bismaleimide and maleic anhydride in a mixed solvent of an aromatic hydrocarbon and an aprotic polar solvent at a low temperature to obtain a bismaleamic acid in accordance with a known process for producing a highly crystalline aromatic bismaleimide (e.g., N,N-(4,4'-diphenylmethane)bismaleimide) (see JP-A-60-260623 and JP-A-63-301226, the term "JP-A" as used herein means an "unexamined published Japanese patent application") and then subjecting the resulting bismaleamic acid to dehydrating cyclization using an acid catalyst (see JP-A-1-211563). According to this process, the cyclization reaction does not proceed sufficiently, and a considerable amount of the bismaleamic acid remains uncyclized in the resulting bismaleimide. If an ether-imide type bismaleimide is crystallized in an aromatic hydrocarbon solvent, the solvent will be included in the precipitated crystals. Hence, for obtaining a high purity product, recrystallization must be conducted in a low-boiling halogenated hydrocarbon solvent (see JP-A-1-238568).
A process comprising dissolving a bismaleimide obtained by drying in tetrahydrofuran or acetone and re-precipitating in water is also known (see JP-A-3-145462). High purity 2,2-bis[4-(4-maleimidophenoxy)phenyl]propane obtained by recrystallization as in the above process is highly crystalline similarly to a highly crystalline aromatic bismaleimide, such as N,N-(4,4'-diphenylmethane)bismaleimide, and therefore has low solubility in a solvent and also a high melting point.
Having excellent heat resistance, a polymaleimide resin is used as a heat resistant modifier for thermosetting resins, such as epoxy resins and vinyl compounds. Where a highly crystalline and high-melting maleimide is used in combination with these reactive liquid resins, it has poor workability in that it is apt to crystallize in the resin solution, requires a high temperature for forming the resin solution, and causes gelation or volatilization of a solvent.